An arm rest for a vehicle seat includes a support bracket (31) to be fixed to a seat back frame (13) of the vehicle seat, the support bracket being formed with a groove (36) at a rear edge side on an upper portion thereof; an arm rest main body (21) rotatably supported to the support bracket (31); and a fitting pin (19) to be provided so as to project from the seat back frame, the fitting pin (19) is fitted into the groove (36) formed in the support bracket (36). The fitting pin (19) is moved backward together with the seat back frame along the groove of the support bracket when an impact stress in a direction where the seat back is inclined backwardly is applied to the seat back.